Amplifier circuit



M. A. LI SSMAN AMPLIFIER CIRCUIT Filed Feb. 6, 1942 Jan. 4, 1944.

3 W0 wvkw Mneaez A. L/SSMA Patented Jan. 4, 1944 UNITED STATES PATENT OFFICE 2,338,342 AMPLIFIER cmcprr Marcel A. Llssman, Temple City, Calif.

Application February 6,1942, Serial No. 429,779

' 2 Claims. (01. 179-171) This invention relates to an improved amplifier circuit whichmay be called a "phase inverter amplifier circuit in that the circuit has a singleended input and a push-pull output.

This circuit is particularly adapted to audiofreqency amplification'and makes it'possible to obtain substantially distortionless power amplification with a minimum number of tubes.

A further object of the invention is to provide a phase inverter amplifier having direct coupling between the output of the single input tube and the input of the push-pull tube.

Another object of the invention is to provide a phase inverter amplifier with constant current type feed-back I p Another'important object of the invention is to provide a phase inverter amplifier with a minimum number of condensers, whereby the frequency response of the amplifier is substantially uniform over a wide range.

The novel features and characteristics of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its construction and mode of operatipn, together with other objects and advantages thereof, will best .be understood by reference to the following description taken in connection with the accompanying drawing disclosing amplifier circuits in accordance with my invention.

Referring to the drawing, Fig. 1 illustrates an amplifier circuit according to my invention;

Fig. 2 illustrates a modification of the circuit of the portion I2 being connected to the cathode 3 and the other end being connected through a grid resistor I4 to the grid 5 of the tube 2. Input terminals I5 and I6 are shown respectively connected to grid 5 of tube 2 through a coupling condenser I0 and to ground. The grid I of the tube I isshown directly connected to the C+ terminal. As will 'become apparent hereinafter, it is generally necessary to raise the grid potential in a positive direction from ground in order to maintain it at the desired negative potential with respect to the cathodewhich is maintained at a higher than normal positive potential by the resistor II. For best results the C+ potential to ground is made equal to the average potential to ground of grid 5 of tube 2.

It may be assumed that there is no direct connection between the input and the grid of tube I. Tube I receives its exciting voltage from the common cathode resistor II, which applies a signalvoltage between the cathode and grid of said tube. The size of the resistor II is suchthat the anode current swing of the tubes I and 2 of Fig. 1, providing a direct coupled .input; and

Fig. 3 illustrates a modification of the circuit of Fig.2. 1 I

Referring to Fig. 1 of the drawing, two electron discharge tubes'are indicated at I and 2 and each comprises at least a cathode 3, an anode I,

and a control grid Ii. A push-pull load device such as center-tapped primary 6 of a push-pull output transformer I is connected between the anodes 4 of the tubes I and 2. Center tap 8 of the primary 6 is shown connected to the B+ terminal. 1

The tubes I and 2 are provided with a common unbypassed cathode resistor II connected at one end to the cathodes 3 and at the other end to ground. In other words, the common cathode return circuit predominantly consists substantially entirely of resistance. The B- and C- terminals are also connected to ground. The resistor II may be considered as comprising two serially connected portions I2 and I3. The resistor portion I2 is of 'sufilcient size to provide the desired biasing voltage on the grid of the tube 2, one end for a given signal voltage is substantially equal. Ordinarily, this means that the resistor II has about three to ten times or more the value of the cathode resistor used for push-pull tubes inconventional circuits. In this manner the current change through resistance II, causing a voltage drop changing the alternating current po-. tential of cathode 3 of tube I with respect to its grid 5, thus exciting this tube, will be only a small fraction of the current change in tube I due to such excitation. Thus, if tubes I and 2 are type 6V6, the resistor would have a value of around 1,000 ohms as compared to the customary value of around 200 ohms. I

A circuit of the type illustrated in Fig. 1 is well adapted to be direct coupled to a preceding stage since the cathodes of the push-pull tubes are operated at a relatively high positive potential with respect to ground. A modification embodying a direct coupled input isillustrated in Fig. 2. The input circuit is shown as comprising a tube 20 having its grid and cathode connected to input terminals 2| and 22. Push-pull output tubes are indicated at 23 and 24 and are shown with their anodes connected to the ends of a centertapped primary 25 of a push-pull output transformer 26. Center tap 21 of the transformer primary is connected to the B+ terminal which is in turn connected through a plate coupling resistor 28 to the anode of the input tube 20. The anode of the input tube 20 is connected directly to the grid of the output tube 23. The cathodes the resistor ii in Fig. 1.

' and to ground through a circuit comprising an unbypassed cathode return resistor 29. B and C-- terminals are connected to ground, and the grid of tube 24 is connected directly to the terminal.

Substantially the same considerations are involved in determining the value of the resistor 29 as are present in determining the value of In Fig. '2 it is not necessary to connect the grid of the tube 23 to an intermediate point on the resistor 29, since purpose, while the bias on the grid of tube 42 is obtained by means of a grid leak return 52 connected to the same bias source as for the the bias for tube 23 is obtained from the plate I of the tube 20. It will be noted that tube is provided with a bypassed cathode return 8|. Other biasing means may be provided, if desired, in order to improve the frequency response of the input. As in Fig. 1, the unbypassed cathode return 29 performs the dual function of biasing the output tubes and acting as a coupling resistor for impressing a signal voltage between the cathode and grid of tube 24. For best results, the C+ potential is made equal to the average potential of the anode of tube 20.

The circuit of Fig. 3 comprises a first tube 40 for voltage amplification, coupled to two power tubes 4| and 42 connected in push-pull by means of a push-pull output transformer 43 to a load L such as a loudspeaker. The cathode of tube 40 is shown grounded. The input to tube 40, which may be obtained from a diode detector, is applied to the grid of tube 40 by means of a potentiometer 44. Unusually satisfactory results are obtained when the input is applied to tube 40 from a known diode biasing arrangement, thus eliminating a coupling condenser which would introduce frequency distortion. Condensers 45, 4'6 and 47 are used to bypass radio frequency, and are sufficiently small not to bypass the highest audio frequencies desired.

With somewhat impaired results, other known means of applying a signal and bias to tube 48 may be used.

The cathodes of power tubes 4| and 42 are connected together, and are connected to ground through a circuit comprising a common unbypassed cathode resistor 48 which provides constant current type negative feed-back as will become apparent. According to this invention, the cathode resistor 48 is made large compared to resistors used in conventional push-pull circuits for self-biasing, in order to obtain the benefits of the invention described below. The cathode resistor 48 is made at least three times, and preferably ten or more times, such conventional value.

A plate coupling resistor 49 is chosen of such value that for the desired average power tube current, the plate voltage of tube 48 for normal operation will be negative with respect to the cathode of tubes 4| and 42, to furnish the proper bias corresponding, to the desired operating point. This can readily .be obtained, because should the measured power tube current exceed the desired value, the coupling resistor 49 can be increased until such desired current is obtained, and vice versa. In practice, it is found grid of tube4l. In the drawing this grid leak return is connected to the plate of tube 48. The value of the resistor 52 is high as compared to the impedance of the condenser 5| at audio frequencies.

In the absence of any signal input, the grids of tubes 4| and 42 will be at the same potential with respect to their cathodes, and each tube will carrythe same current. When an audio frequency signal is applied to the input, the audio-frequency voltage madeavailable between the grid of tube 4| and ground may be considered as applied to the tubes 4| and 42 in series, and will divide between these tubes in such a way as,to straighten the tube characteristics. I

This can be illustrated as follows: When a positive voltage is applied to the grid of tube 4|, the current in tube 4| increases, causing an increased drop in resistor 48, which will apply a negative voltage between the grid and cathode of tube 42. Due to the curvature of the tube characteristics, the current increases faster for a given positive voltage than it decreases for a corresponding negative voltage, especially for large voltage swings. For a linear characteristic the available voltage would divide equally between the tubes 4| and 42. When the grid of tube 4| is positive, tube 4| will carry an excess of current, resulting in .an additional drop in resistor 48, so that less than half the available voltage will be applied to tube 4| in the positive direction, and more than half the available voltage will be applied to tube 42 in the negative direction. The increase in current in tube 4| will then be more nearly equal to the decrease in current in tube 42.

On the other hand, when the applied voltage to tube 4| is negative, the change in current in tube 4| will be less than corresponding to a linear characteristic, thus more of the available voltage will be applied to tube 4| in'the negative direction than to tube 42 in the positive direction. Thus, the division of the available voltage between tubes 4| and 42 is always in the direction to straighten the tube characteristics.

When the cathode resistor 48 is made large compared to the conventional value for selfbiasing, then the change in total current taken by the two output tubes will be small compared to the change in current in the individual tubes. Furthermore, this change in total current is blocked from reaching the load due to the pushpull connection.

It will be observed that no amplification is lost by making-the cathode resistor 48 large, as the "available voltage is always divided between tubes 4| and 42. However, by making resistor 48 large, any unwanted'response in the power tubes is highly suppressed. This is because anycurrent component not corresponding to an applied voltage component will result in a voltage drop in resistor 48 applying a voltage to the tube in a. direction to suppress the undesired current component.

The power loss in the cathode resistor 48'is obtained from the Bsupply, and when tubes of high power sensitivity are used, such loss is not found to be objectionable. Very satisfactory results have been obtained by using the triode section of a 6SQ7 type tube for tube 40 and two tubes of the 6V6 type for tubes 4| and 42 with resistors 49 and 48 equal to one megohm and 1,000 ohms, respectively.

It should be noted that the explanation of the operation of the circuit based on the application of the signal voltage between one of the grids and ground in the push-pull stage as described in considerable detail in Fig. 3 may be applied with equal force to the description of the operation of the circuits illustrated in Figs. 1 and 2. At the same time the explanation of operation advanced inconnection with Figs. 1 and 2 applies withequal force to Fig. 3.

While I have shown and described a preferred embodiment of the invention, it will be understood that modifications and changesmay be made without departing from the spirit and scope of the invention, as will be understood by those skilled in the .art.

I claim:

1. In an audio-frequency amplifier circuit of the push-pull output type, in which a pair of.

electron discharge devices each provided with at least an anode, acathode and a control grid, have their anodes connected to the respective opposite ends of a center-tapped push-pull load device, said circuit further including an anode supply having a positive terminal connected to the center tap and a negative terminal, the combination which comprises: a capacity connected between one of the grids and-the negative terminal; a. resistor connected between said grids, said resistor having a relatively high value as compared to the impedance of said capacity at audio frequencies; a common unbypassed bias resistor connected in circuit between the cathodes and .the negative terminal; and input circuit means for impressing a signal voltage between the other of said grids and said negative terminal.

2. In an amplifier, the combination which comprises: a first and a second electron discharge device each having at least an anode, a cathode and a control" grid; a center-tapped load device connected between said anodes; an anode supply source having its positive terminal con-' nected to said center tap; a bias resistor having an end connected to the negative terminal of the anode supply source; means connecting the cathodes together and to the other end of the bias resistor; a. resistor connected between MARCEL A. LISSMAN. 

